By means of immunocytochemistry we have investigated subsets of T lymphocytes in frozen sections of nasal mucosa from patients with seasonal allergic rhinitis and healthy control persons. All participants were subjected to time-course provocation during the non-pollen season, and samples were taken during provocation as well as during the natural pollen season. Computerized image analysis was applied for evaluation of the immunostained lymphocytes. CD45RO+ memory T cells outnumbered the remaining leukocyte populations in the mucosa of both allergic patients and controls on all occasions. During the repeat provocation there was no difference in numerical values, with respect to any of the five leukocyte subpopulations studied (CD4, CD8, CD25, CD45RA and CD45RO), between patients and controls. However, during continuous exposure in the pollen season a significant increase in CD4+ cells was observed in allergic patients compared to before provocation (p<0.05). No changes were observed with respect to CD8+ and CD25+ cells. Similarly, an increase in CD45RO+ memory was found in allergic patients during the pollen season compared to the non-pollen season (p<0.02). This latter finding was, however, only evident in the patients who did not use nasal corticosteroids. Hence the present investigation has demonstrated an allergen-induced increase in CD4+ and CD45RO+ memory T cells in the mucosa of allergic patients during the pollen season. These events may constitute a cellular basis for local continuous production of certain cytokines, particularly interleukin-4, which is essential for IgE synthesis.